1. Field of the Invention
The invention concerns a process for the manufacture of an exhaust silencer.
In order to reduce the noise emitted by the exhaust systems of motor vehicles, the exhaust pipe is provided along part of its length with perforations around its circumference through which the vibrational energy of the exhaust gases can escape into the surrounding space. This space is enclosed along the length of the perforated pipe section by a sealed silencer casing containing a packing of mineral wool, the sound damping effect of which nullifies the vibrational energy escaping from the exhaust pipe.
2. Description of the Related Art
An obvious process for introducing the mineral wool packing into the silencer casing consists of pushing loose mineral wool between the exhaust pipe and the silencer casing and then finally sealing the silencer casing. The disadvantage here, however, is that the introduction of the loose mineral wool on the part of the silencer manufacturer involves a not inconsiderable amount of work which has an adverse effect on production times; in the cast of mass products such as these manufactured for bulk buyers, namely the automobile industry, such labor-intensive and time-consuming manufacturing processes bring a substantial and adverse influence to bear. Moreover, introduction of the required amount of mineral wool and sufficiently uniform distribution of density cannot be reliably assured and are, rather, dependent upon the skill of the assembly personnel.
For these reasons, a change of practice has taken place whereby the mineral wool is prepared by the mineral wool manufacturer in the shape required for the packing operation, in which form it is then supplied to the silencer manufacturer who then inserts these shape-stabilized packings into the silencer casing.
There are various processes available for stabilizing the shape of such mineral wool packings, involving for example the wrapping of loose mineral wool in a thin sheeting and/or quilt-stitching. One method currently being applied consists of the manufacturer providing the mineral wool with a bonding agent and then curing this, thus stabilizing the shape of the mineral wool element.
The widely applied practice in this case involves the employment of a synthetic resin bonded board from which the required shape is milled. This produces shapes of very accurate dimensions, thus minimizing disruption to the production process of the silencer manufacturer. A further advantage of this process consists in the fact that the process for manufacture, including the hardening of the boards of mineral wool, can be implemented on a large scale in the usual way without any additional expenditure, by curing the bonding agent employed for stabilizing the shape of the boards in the tunnel drier of the production line belt, so that only the milling operation adds time to the usual felt web production process. However, this process has the disadvantage that the milling operation produces a not inconsiderable amount of waste material which, although it can be re-melted for re-use, has the effect of increasing the cost of the shape-stabilized element manufacturing process.
Moreover, a process is also known from German patent application DE-OS 32 05 186 whereby the mineral wool provided with bonding agent which has not yet been cured is moulded around a core, the shape of which corresponds to that of the exhaust pipe, and then subsequently enclosed by a moulding shell corresponding to the internal contour of the silencer casing, in which position the bonding agent curing process is allowed to take place. When the moulding shell has been opened and the core removed, a single-piece moulding is left in the shape of the required silencer packing, which can then be supplied to the silencer manufacturer and placed by him onto the actual exhaust pipe for insertion into the silencer. One disadvantage of this process lies in the fact that the introduction of the mineral wool into the mould, and mould removal involves a considerable amount of work, and substantial additional time is required for the curing process unless hardening in the tunnel drier is integrated within the production process in the same way as in the case of the manufacture of felts and boards for standard products. The curing process in the special mould thus requires further specific investment and additional energy input, and may also hinder the production process of the mineral wool manufacturer.
This process is, furthermore, only possible if the exhaust pipe, and thus the core corresponding in shape to the exhaust pipe, has the shape of a simple cylinder, so that the moulding, following curing, can be withdrawn and subsequently fitted without difficulty. In the case of bent, bifurcated or otherwise irregularly shaped exhaust pipes in the area of the silencer, this process cannot be employed. In this connection it is known from the German patent specification DE-PS 32 38 638, that the original internal component of the silencer comprising the exhaust pipe and corresponding ancillary elements such as partitions or similar, may be provided by the mineral wool manufacturer with pads or pre-resined but not yet cured mineral wool and, thus prepared, placed in a moulding shell for curing. The mineral wool may also be applied somewhat more thickly to the internal component as it yields prior to curing under the pressure of the subsequently fitted moulding shell. Following the curing process, the composite component formed from the internal component and the cured mineral wool, is removed from the moulding shell and supplied in this condition to the silencer manufacturer who then merely has to insert this composite component laterally into the silencer casing proper.
This process is even more expensive and comprises the following individual manufacturing steps:
1. supply of the finished internal components for the silencer by the silencer manufacturer to the mineral wool manufacturer.
2. manufacture of mineral wool webs with additional application of a synthetic resin bonding agent,
3. production of a large number of pads e.g. by tearing or cutting the required shapes from the mineral wool webs,
4. introduction of the pads and the original internal component into a moulding shell,
5. curing of the bonding agent in a heated facility,
6. de-moulding of the finished composite components,
7. delivery of the composite components comprising the internal components and the cured mineral wool by the mineral wool manufacturer back to the silencer manufacturer.
As is immediately apparent, the considerable expenditure already required for the manufacture of so-called wrap mouldings in accordance with DE-OS 32 05 186 is increased even further by the fact that the introduction of a large number of small individual pads is more labor-intensive than is the wrapping of a straight pipe with a relatively long mineral wool web. Moreover, particularly considerable additional transportation costs are incurred as each original internal component has to be first transported from the silencer manufacturer to the mineral wool manufacturer, and then, following formation of the composite component, from the latter back to the silencer manufacturer.